This invention relates to the preparation of brazeable metals for brazing and in particular to the joining of braze alloy to parts to be brazed. It is known to supply brazeable parts having a coating or cladding of a braze alloy. A characteristic of the cladding alloy is that it will melt at a temperature lower than the melting temperature of the parent material. In the brazing process it flows to form a coherent joint between the clad part and adjoining parts at a temperature which approaches but is still less than the melting temperature of the parent material. The term "brazing sheet" is frequently used to describe pre-made parts comprising a base sheet of a parent metal, either one or both sides of which is overlaid by a cladding of braze alloy. Purchased braze sheet may be cut, sized and otherwise machined prior to being assembled with other parts and united therewith in a brazing process.
The cladding or braze alloy is itself produced in a solid, sheet-like form. In one method of fabrication of brazing sheet, a sheet of brazing alloy is super-imposed on a base sheet of a parent metal, either to one or both sides thereof, with the assembly so formed being then subjected to a roll bond or like operation in which the overlying alloy sheet is made mechanically to adhere to the underlying base sheet. This, and similar joining techniques, has the disadvantage of being inapplicable to irregularly shaped parts which cannot be conventionally clad and then bent to a desired configuration. Moreover, brazing sheet normally is made with standard thicknesses of brazing alloy, with an absence of close dimensional tolerance as may be desirable for subsequent machining and fixturing.
A problem of brazing irregularly shaped parts can be dealt with to a limited extent by applying the braze alloy as a poured molten metal, as a compacted powder and as a foil. All have certain disadvantages which become particularly apparent when a part is required to be machined prior to brazing. In the making of header plates for tubular heat exchangers, for example, the plates are required to be drilled so that tubes may be installed, and, in a subsequent brazing operation, unitarily joined to the plate. Flat recessed surfaces of the plate become reference surfaces for drilling operations. Poured molten metal on recessed surfaces makes them rough and uneven and therefore unsuitable as reference surfaces. Compacted powders, even though sintered, are relatively porous. They absorb lubricants during the drilling process with the result that the making of a sound, uniform bond between the sintered braze alloy and parent metal is inhibited. Application of the braze alloy in a foil form has aspects of practicality but to avoid the problem of joint contamination by drilling lubricants, and to insure accurate drilling, it is necessary separately to drill the foil and the header plate and then to superimpose the foil on the plate so that drilled holes align with one another. This is a relatively costly assembly process, however, and the obtaining of precise alignment of drilled holes may be difficult.